Compound Interest Calculator

How to use the Compound Interest Calculator

Enter the amount you plan to start with, choose whether to add regular contributions, and set an annual interest rate. Then pick how often interest compounds, how long the money grows, and the currency you want results in. Results update live as you type.

1. Starting amount: the balance you begin with. Enter 0 to see the effect of contributions alone.
2. Add contributions: turn this on to add regular payments. Choose the contribution frequency (weekly, biweekly, semi-monthly, monthly, quarterly, or yearly), whether each payment lands at the start or end of the period, and an optional annual step-up that grows your contribution each year.
3. Annual interest rate: the nominal rate before compounding, between 0 and 50%.
4. Time period: enter years and months. Fractions such as 2.5 years are also accepted.
5. Compounding frequency: how often interest is added, from continuous compounding down to once a year.
6. Currency: pick from 17 currencies, including USD, EUR, GBP, JPY, and INR. Results are formatted with the correct symbol and decimal style.
7. Optional adjustments: add an inflation rate to see the future value in today’s money, and a tax rate to see the effect of tax on the interest you earn.

To find out how much you need to save each period to hit a target, turn on Calculate a contribution goal, enter the target future value, and the calculator works out the required contribution for you.

What the results mean

• Future value: the total balance at the end of the period, including your starting amount, all contributions, and all compound interest.
• Required contribution: shown in goal mode, this is the amount you need to save per period to reach the target.
• Total contributed: your starting amount plus every contribution.
• Total interest earned: the difference between the future value and the total you contributed.
• Effective annual rate (APY): the actual yearly return after compounding, which is higher than the nominal rate when interest compounds more than once a year.
• Growth: the percentage increase of your money relative to what you put in.
• Rule of 72: a quick estimate of how many years it takes to double your money, found by dividing 72 by the APY.
• Time to double: the exact number of years needed to double your balance at the effective annual rate.
• Inflation-adjusted value: the future value expressed in today’s purchasing power, shown when you enter an inflation rate.
• After tax: the future value minus tax charged on the interest earned, shown when you enter a tax rate.
• With simple interest: what the balance would be if only your starting amount earned interest and contributions earned nothing.
• Extra from compounding: how much more you earn from compound interest compared with the simple interest result.

The growth chart plots your balance, your cumulative contributions, and the inflation-adjusted balance over time. Open the year-by-year breakdown to see how the balance builds each year, and use Export CSV to save the table as a spreadsheet.

How the math works

The calculator converts your nominal annual rate into an effective annual rate based on the compounding frequency. When interest compounds n times per year at a nominal rate r, the effective annual rate is:

(1 + r / n)ⁿ − 1

With continuous compounding, the effective annual rate is e^r − 1, where e is Euler’s number. The effective annual rate is then divided across the contribution periods so that contributions grow at the same effective pace. The starting amount grows over the whole period, and each contribution earns compound interest from the moment it is made.

Worked examples

• Start with $10,000 at 7% for 20 years with monthly compounding. The future value is $40,387.39, of which $30,387.39 is interest.
• Add a $300 monthly contribution (paid at the end of each month) to the same plan. The future value rises to $196,665.39, with $82,000 contributed and $114,665.39 earned in interest, a growth of about 140%.
• Switch the same plan to daily compounding and the future value becomes $197,200.45, slightly higher than the monthly compounding result.
• Use goal mode to reach $500,000 in 20 years from a $10,000 start at 7%. You need to save about $882.30 per month.
• Apply 3% inflation to the first example. The $40,387.39 future value is worth about $22,361.52 in today’s money. Apply 20% tax to the interest and the after-tax result is about $34,309.91.
• Compare the $300 monthly plan with simple interest. Simple interest gives $96,000, so compounding earns an extra $100,665.39.

Compounding frequencies explained

• Continuously: interest is added at every instant, the theoretical maximum for a given nominal rate.
• Daily, weekly, biweekly, and semi-monthly: frequent compounding that adds a small advantage over monthly.
• Monthly: a common choice for savings accounts and loans.
• Quarterly, semi-annually, and annually: less frequent compounding that lowers the effective return.

Limits and notes

• The annual interest rate must be between 0 and 50%.
• The time period must be between 0.5 and 100 years.
• The inflation rate must be between 0 and 20%, and the tax rate between 0 and 100%.
• The annual step-up on contributions must be between 0 and 50%.
• Inflation and tax are applied as simple end-of-period adjustments and do not compound year by year.
• Results are estimates for planning purposes and do not include account fees, withdrawal penalties, or market volatility.